NATO: Russian troops could move quickly

Malaysia Airlines jet crashes in Ukraine – Debris from Malaysia Airlines Flight 17 sits in a field at the crash site in Hrabove, Ukraine, on Tuesday, September 9. The Boeing 777 is believed to have been shot down July 17 in an area of eastern Ukraine controlled by pro-Russian rebels.

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Malaysia Airlines jet crashes in Ukraine – Australian and Dutch experts examine the area of the crash on Sunday, August 3.

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Malaysia Airlines jet crashes in Ukraine – A woman walks with her bicycle near the crash site on Saturday, August 2.

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Malaysia Airlines jet crashes in Ukraine – Police secure a refrigerated train loaded with bodies of passengers from Malaysia Airlines Flight 17 as it arrives in a Kharkiv, Ukraine, factory on Tuesday, July 22.

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Malaysia Airlines jet crashes in Ukraine – A pro-Russian rebel passes wreckage from the crashed jet near Hrabove on Monday, July 21.

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Malaysia Airlines jet crashes in Ukraine – Wreckage from the jet lies in grass near Hrabove on July 21.

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Malaysia Airlines jet crashes in Ukraine – A man covers his face with a rag as members of the Organization for Security and Co-operation in Europe and the Dutch National Forensic Investigations Team inspect bodies in a refrigerated train near the crash site in eastern Ukraine on July 21.

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Malaysia Airlines jet crashes in Ukraine – Emergency workers carry a victim's body in a bag at the crash site on July 21.

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Malaysia Airlines jet crashes in Ukraine – A piece of the plane lies in the grass in eastern Ukraine's Donetsk region on July 21.

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Malaysia Airlines jet crashes in Ukraine – An armed pro-Russian rebel stands guard next to a refrigerated train loaded with bodies in Torez, Ukraine, on Sunday, July 20.

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Malaysia Airlines jet crashes in Ukraine – Ukrainian State Emergency Service employees sort through debris on July 20 as they work to locate the deceased.

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Malaysia Airlines jet crashes in Ukraine – A woman covers her mouth with a piece of fabric July 20 to ward off smells from railway cars that reportedly contained passengers' bodies.

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Malaysia Airlines jet crashes in Ukraine – Toys and flowers sit on the charred fuselage of the jet as a memorial on July 20.

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Malaysia Airlines jet crashes in Ukraine – People search a wheat field for remains in the area of the crash site on July 20.

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Malaysia Airlines jet crashes in Ukraine – A woman walks among charred debris at the crash site on July 20.

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Malaysia Airlines jet crashes in Ukraine – Emergency workers load the body of a victim onto a truck at the crash site on Saturday, July 19.

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Malaysia Airlines jet crashes in Ukraine – Emergency workers carry the body of a victim at the crash site on July 19.

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Malaysia Airlines jet crashes in Ukraine – A large piece of the main cabin is under guard at the crash site on July 19.

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Malaysia Airlines jet crashes in Ukraine – Victims' bodies are placed by the side of the road on July 19 as recovery efforts continue at the crash site. International officials lament the lack of a secured perimeter.

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Malaysia Airlines jet crashes in Ukraine – A man looks through the debris at the crash site on July 19.

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Malaysia Airlines jet crashes in Ukraine – An envelope bearing the Malaysia Airlines logo is seen at the crash site on July 19.

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Malaysia Airlines jet crashes in Ukraine – Armed rebels walk past large pieces of the Boeing 777 on July 19.

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Malaysia Airlines jet crashes in Ukraine – Ukrainian rescue workers walk through a wheat field with a stretcher as they collect the bodies of victims on July 19.

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Malaysia Airlines jet crashes in Ukraine – A woman looks at wreckage on July 19.

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Malaysia Airlines jet crashes in Ukraine – Pro-Russian rebels stand guard as the Organization for Security and Co-operation in Europe delegation arrives at the crash site on Friday, July 18.

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Malaysia Airlines jet crashes in Ukraine – A woman walks through the debris field on July 18.

Malaysia Airlines jet crashes in Ukraine – Wreckage from Flight 17 lies in a field in Shaktarsk, Ukraine, on July 18.

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Malaysia Airlines jet crashes in Ukraine – A man covers a body with a plastic sheet near the crash site on July 18. The passengers and crew hailed from all over the world, including Australia, Indonesia, Malaysia, Germany and Canada.

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Malaysia Airlines jet crashes in Ukraine – A diver searches for the jet's flight data recorders on July 18.

Violence on the rise in Donetsk

Just Watched

Reporting in Ukraine as fighters close in

And small-arms fire came very close to the investigation team at the crash site Wednesday, forcing them to withdraw from the site, said Pieter-Jaap Aalbersberg, head of the recovery mission.

Searchers are disappointed they haven't been able to complete their mission, he said. But he said they did make significant headway, recovering human remains and personal belongings like photo albums, diaries and toys.

"We aim to return to the crash site as soon as we have the opportunity to work there for a longer period under stable conditions," he said.

Uptick in Donetsk violence

Ukrainian forces continued their attempt to take back control of the restive city of Donetsk from pro-Russian separatists Wednesday.

Russia's Foreign Ministry claimed in a statement Monday that the Ukrainian military was firing missiles and using multiple rocket systems in and around the city.

It accused Ukraine's government of wanting to continue the war and called for talks to find a political solution to the crisis.

With escalated fighting and Ukrainian forces making gains, the Russian deployment at the border could portend an intervention under the banner of a peacekeeping operation.

Russia asked the U.N. Security Council on Tuesday for an emergency hearing on the humanitarian crisis in Ukraine

According to a NATO official, Russia now has about 20,000 troops stationed "in an area along the entire border with eastern Ukraine." The buildup nearly doubled the troop deployment in the past week by adding 8,000 more forces to 12,000 already there, the official said.

In addition, Russia's Defense Ministry is staging a week of military exercises involving air troops and anti-missile defense forces. The exercises are taking place in Russia's southern Astrakhan region, roughly 500 miles from the border with Ukraine.

Similar military exercises preceded Russia's annexation of Crimea in March, which escalated the Ukraine conflict after the ouster of pro-Moscow President Viktor Yanukovych a month earlier.

Refugees flee conflict

The ongoing fighting has killed close to 1,400 people -- civilians and combatants -- and more than 4,000 people have been wounded in eastern Ukraine since mid-April, according to U.N. officials.

The fighting has also forced hundreds of thousands of people to flee their homes and seek shelter either elsewhere in Ukraine or across the border in Russia.

The U.N. refugee agency, the UNHCR, said Tuesday that the Russian authorities estimate that around 730,000 Ukrainians have sought sanctuary in Russia this year under a visa-free travel program. This includes more than 168,000 who have reported to Russia's Federal Migration Service.

UNHCR's European director, Vincent Cochetel, told journalists in Geneva that about 80% of the Ukrainians who have fled to Russia are staying in border areas, while others are with friends or family in other parts of the country.

Many people in eastern Ukraine are Russian-speaking and have strong ties to Russia.

At the same time, nearly 118,000 people are reported to be internally displaced, most of them from eastern Ukraine, based on information given to the UNHCR by Ukrainian authorities and local NGOs.

"In the past seven days more than 6,200 people have been forced from their homes," Cochetel said.

Many have fled with limited possessions, some driven out by the destruction of homes and infrastructure, while others say they fear being caught in the crossfire or suffering persecution.

John Ging, operations director of the U.N. Office for the Coordination of Humanitarian Affairs, called on all sides Tuesday to allow people to move freely and safely.

"The humanitarian situation is deteriorating, and the worrying increase in violence in urban areas puts a greater number of people at risk.

"Until violence is ended, we will continue to see an increase in human suffering, and in the number of people displaced. Immediate action is required to prevent this crisis from worsening," he said, according to prepared remarks.